The present invention relates to an improvement in a method for observing a reaction process by the transmission electron microscopy.
For observation of material structure by the transmission electron microscopy, a sample cut out of the material needs to be processed into a small thickness on the order of several tens to several hundred nm that enables the sample to transmit an electron beam.
A method of preparing transmission electron microscopic samples for observation under heating is presented, for example, in a literature "DENSHI KENBIKYO GIHO (Electron Microscopy), edited by Electron Microscopy Society of Japan, Kanto Branch, published by Asakura Shoten, 1991, pp. 119-130". In this method of preparing transmission electron microscopic samples, a sample piece cut out in a specified size is subjected to a mechanical grinding process so that its center becomes the thinnest. The sample piece is further subjected to a thinning process by ion milling or chemical etching. This processing is completed when the sample gets partly holed. The sample obtained in this way is observed by transmission electron microscopy in a wide area around the hole wherein the sample has a thickness of several tens to several hundred nm.
The above sample preparing method is described in detail with reference to FIGS. 6A, 6B, 7, 8A, 8B, 9A, 9B, 10A and 10B.
As shown in FIGS. 6A and 6B, a rectangular piece 24 of a 1 mm-3 mm square is cut out of a wafer 21. Reference numeral 22 denotes a substrate, and 23 denotes a surface of the piece prior to heat treatment. Two such pieces 24 are prepared. Then, as shown in FIG. 7, the two pieces 24, 24 are bonded together by thermosetting resin 25 with their surfaces 23 facing each other, and then baked for about 3 hours at 180.degree. C. As a result, a sample 26 is produced.
Next, as shown in FIG. 8A, the sample 26 is attached to a bottom surface of a weight 27a of a grinding tool 27, with the bonded surfaces of the sample 26 being vertical to the weight bottom surface (see FIG. 8B). Then, the sample 26 is ground to a thickness of about 100 .mu.m by rotating a rotary grinder 29 while an abrasive 28 is being fed. In this process, the sample 26 is abraded by successively changing the type of the abrasive 28 and of the rotary grinder 29 until a ground surface has no flaws. In this way, the surface of the sample 26 gradually changes from a coarse state to a fine state.
Next, as shown in FIG. 9A, the sample 26 is fixed on a sample mount 30 of a dimple grinder, facing down the surface 5 of the sample 26 ground by the rotary grinder 29. Then, a region 32 around the bonded part is ground into a mortar shape by a dimpler 31, as shown in FIG. 9B and FIG. 9C (a sectional view as indicated by arrows C--C of FIG. 9B).
Next, as shown in FIG. 10A, an ion beam 33 is applied to the opposite surfaces of the sample 26 by ion milling equipment. Thus, the sample 26 is ion milled to be thinned until a central portion of the area 32 has a thickness on the order of several tens to several hundred nm. The reason for using the ion milling technique to process the sample into a thickness on the order of several tens to several hundred nm is that there is a limit in thinning the sample through mechanical polishing by the rotary grinder 29, dimple grinder or the like.
The sample 26 thus obtained, shown in FIG. 10B, is fixed to a heatable holder of a transmission electron microscope. Then, changes in the state at the bonded part 34 are observed while heating is effected (with heating conditions changed).
Another method of preparing a sample for observation by the transmission electron microscopy different from the foregoing sample preparing method is described in Japanese Patent Laid-Open Publication HEI 5-180739. In this sample preparing method, a piece of specified dimensions is cut out of a material, and then the piece is subjected to a mechanical cutting process so as to be formed into an L-shape or a convex shape in cross section with its observation part left. After that, the observation part is further thinned by a convergent charged-particle beam, whereby a sample having a thickness of 0.1 .mu.m is obtained.
The sample obtained in this way is fixed to a heatable holder of a transmission electron microscope. Then, changes in the state of the material piece around the observation surface is observed while heating is effected (with heating conditions changed).
However, when reaction process is observed with the sample prepared by either of the above conventional methods, there arises a problem as described below.
That is, for transmission electron microscopic observation of state changes of a material under heating, it is necessary to process the sample to a small thickness on the order of several tens to several hundred nm such that the sample can transmit an electron beam. Such a small thickness of the sample increases a ratio of cross sectional area to volume of a region where changes in state occur when heated, and also increases the area of contact with the vacuum. Thus, the heating of the sample having a film thickness small enough to transmit the electron beam would involve conditions different from the conditions in heating a sample in a wafer state as shown in FIG. 6A or a sample having a film thickness that is too large to transmit the electron beam.
Accordingly, with the experimental sample as prepared by either of the above conventional methods, a reaction process similar to that which actually occurs in a wafer state would not be able to be observed.
Thus, it could be conceived that after a thick-film sample is formed and heated, the sample is processed to such a thickness as to transmit the electron beam, and then observed by a transmission electron microscope. However, in this case, although observation of the state after the heating process is possible, observation of the state during the heating (i.e., an intermediate state) is impossible.
Of course, if a large number of wafers or thick-film samples that have been heated at various temperatures are prepared and processed into a thickness small enough to transmit the electron beam and then observed by a transmission electron microscope, intermediate states will be observed. Unfortunately, however, there arises a problem that numbers of samples would be needed. Further, there is another problem that because the heating temperature cannot be varied continuously, there may be cases in which changes in reaction process at certain temperatures cannot be observed.